One way of mixing (that I try to follow) is to do ALL of the mixing in mono. EQ settings, automation, compression, etc. etc. Make it sound good in mono. Because you never know when you'll have playback in mono. The more important thing though, is that when you get things as clear sounding as you can in mono, when you pan instruments, things become even more clear, and you get great instrument seperation going on.
Unfortunately, for my album, my stereo recorded instruments (drums, piano, acoustic guitar) sound great panned hard l/r but have phase issues when in mono, so I have to mix with them seperated. All my other instruments are mixed initially in mono though, so at least I have that advantage.
After you pan, re-adjust all your settings as well.